The present invention is directed to a method of ultrasmooth grinding of tubes of a zirconium-based alloy. The zirconium-based alloy can be used for cladding of fuel in thermal rectors. The method includes conventional manufacturing of tubes from the stadium of ingot to the final annealed tubes, ultrasmooth grinding and after that non-destructive and destructive testing according to conventional techniques. The method can be used in the manufacture of cladding tubes for fuel rods for boiling water reactors and compressed water reactors. Additionally, the invention has application in the area of construction tubes, such as guide thimble tubes, and instrumentation tubes for fuel rods for PWR and construction tubes for fuel rods for BWR.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Among the alloys that are most customarily used in the manufacture of cladding tubes for fuel rods for boiling water reactors (BWR) and compressed water reactors (PWR), alloys such as xe2x80x9cZircaloyxe2x80x9d are especially known, amongst which Zircaloy-2 and Zircaloy-4 have the following composition, in accordance with ASTMB 811:
In addition to the above, the content of oxygen for these alloys should be between 900 and 1600 ppm for cladding tubes and construction tubes for fuel element skeletons. Furthermore, Zircaloy-2 and Zircaloy-4 are commercially used with insignificant, but well-defined additives of Si and/or C, preferably in contents of 50-120 ppm Si and 80-270 ppm C.
These zirconium-based alloys are chosen because of their nuclear properties, foremost being the low neutron absorption cross-section, which imparts a low ability to absorb neutrons, mechanical properties, and corrosion resistance in water and steam at high temperatures and under radiation.
The development goes to meet longer service times for fuel rods, i.e., to allow a higher extent of rates of burn-up. It has been proved thereby to be of importance to be able to improve the corrosion resistance to not exceed the design related maximum oxide thickness of the zirconium-base alloyed fuel rods. Furthermore, a very important factor is to minimize the activity build-up on the fuel, i.e., on the outer surface of the fuel rods. Since the service time is extended, there are higher demands made on the outer surface of the fuel rods, such as in reference to their condition and surface roughness. Experience has shown that a pickled and preautoclaved surface with fine surface roughness reduces the contribution to deposition of activity-contributing elements on the surface of the fuel rod as compared to fuel rods with conventional surface finishes with a coarser surface roughness.
It has been a widely used technique to manufacture cladding and construction tubes of zirconium alloys by a manufacturing process that includes hot working of an ingot to a bar, quenching from the xcex2 area, machining to a hollowed work piece, extrusion of the hollowed workpiece in a xcex1-area to a tube hollow, reducing in several operations by cold rolling to substantially final dimension with a recrystallization annealing in the xcex1-area for each cold rolling operation. The cold rolled tube, substantially in the final dimension, will be vacuum annealed in the xcex1-area at a temperature in the interval of 450-650xc2x0 C. and surface-conditioned by some of the methods blasting or pickling. Alternatively, a combination of these methods can be used. Also, the tube can be surface-conditioned on the outside by mechanical treatment in form of one, or a combination of, the methods belt grinding, belt polishing, contact grinding and polishing.
The object of the present invention is to provide an improved method for the manufacturing of tubes of zirconium alloys for cladding of fuel and for construction of fuel rods for skeletons, including mechanical treatment of the outer surface that, after the final vacuum annealing, meet the requirements for fine surface roughness in order to minimize activity build-up. Certainly, a fine surface roughness (and by that, a low activity build-up) as above mentioned could be performed by which the outer surface of the tubes after the final vacuum annealing will be pickled and preautoclaved in high-pressure steam. However, several disadvantages are found with this method. At first the pickling must be done in a mixture of nitric acid and hydrofluoric acid. This mixture is disadvantageous in regards to the working environment and also the handling of the consumed acid, which can contribute to an unwanted increase of the environmental pollution. At second the autoclaving contributes to corrosion-produced hydrogen, which will be absorbed by the zirconium alloys. This causes the cladding and construction tubes to have an undesirable shortening of the total operating time, since the autoclave produced hydrogen adds to the hydrogen that will be absorbed during operation.
A method is provided for manufacturing tubes especially for fuel elements and tubes for construction of fuel rod skeletons made of zirconium-based alloy for nuclear reactors. Tubes made by the method are ground to a surface roughness of xe2x89xa60.2 xcexcmRa with a belt device of SiC-based belts, so-called xe2x80x9cdressedxe2x80x9d SiC-based belts with cotton back or SiC-cork belts. The tubes may possibly be polished after grinding with a wheel to a surface roughness xe2x89xa60.25 xcexcmRa.